This invention relates to a case for packaging, widely used as a disposable lunchbox and for food-packaging cases in supermarkets, fast food outlets and other food shops.
This type of case for packaging, seen in the prior art, consists of a single-layer or multiple-layer plastic sheet of polyethylene, polypropylene, polyvinyl chloride, etc., whether semi-hard or hard, solid-wall or foamed-wall. Any of these raw materials is formed by vacuum forming, pressure forming, hot press or another suitable sheet-forming method.
Some of these cases for packaging, seen in the prior art, are formed to a specified shape by sheet-forming a raw material or water/oil-proof paper, using a hot press, etc.
Another type of packing case, used in the prior art, is formed by injecting a plastic into a thin-walled structure.
A case of such a type may be with or without lid.
A case with a lid is classified into a separate type and a connected type; in the former, the lid is completely separated from the case body and, in the latter, the lid 2 is connected to the case body 1 through a hinge 3 as shown in FIG. 3(a).
A practical example of the latter is described as follows. The case comprises a case body 1 and a lid 2, which is connected to the case body through a hinge 3 provided on one side with an open edge of the case body. An engaging tab 4 and a slot 5 are provided oppositely on the lid edge 21 and the case body edge 11 on the opposite side to the hinge.
In order to close and fix the lid 2 onto the case body 1, the lid is closed against the reactive force of the hinge 3 while the engaging tab 4 is bent downwards and inserted into the slot 5 (FIG. 3(b)).
This type of case for packaging is manufactured by forming a plastic sheet by either vaccum forming, pressure forming, etc., and then punching and trimming.
However, such a type of packaging case seen in the prior art has the following disadvantages.
With this example shown in the prior art, the engaging tab 4 is formed in a rectangular shape and inserted into the slot as cut in a straight line. Therefore, the fixing force of the lid 2 is so weak that, after the lid is closed, the engaging tab gradually come off while being driven by the counterforce of the hinge 3 against denting, thus the lid 2 often opens naturally. Therefore, the lid sometimes opens naturally even after being fixed.
In order to solve this problem, a case shown in FIGS. 3 (c) and (d) is proposed as a state-of-the-art development.
With a case shown in FIG. 3(c), a hook 41 is formed, laterally protruding from the top edge of the side of the engaging tab 4. When the tab is inserted in the slot 5, the hook is engaged with the edge of the slot for preventing disengagement of the tab.
According to this example of the prior art however, inserting and engaging operations are slightly time consuming to connect the tab 4 into the slot 5. In the extreme, the book 41 was often broken at the root during engaging operations, resulting in easy disengagement of the tab.
Another example of the prior art is shown in FIG. 3(d), in which an engaging tab 4 is formed substantially in a dovetail, whereby this tab is prevented from easy disengagement. With this example however, operations for insertion and engaging the engaging tab 4 were slightly laborious and time consuming.
Furthermore, another example of the prior art is shown in FIGS. 3 (e).about.(h), where an engaging tab 4 is provided, protruding outwards from one side of the case body 1, while a side slit is equipped on the edge of one side of the lid 2 corresponding to this tab, thereby the tab is configured to enter the slot easily.
With this example seen in the prior art, operations for closing the lid 2 and fixing it to the case body 1 are carried out as follows. When the lid 2 is closed first, the lower face of the lower edge 51 in the slot 5 comes in contact and is supported by the upper face of the tab 4 (FIG. 3(f)). When the side wall on the tab 4 side of the case body 1 is deflected by pressing with a fingertip toward the inside of the case body as shown by the chain line in FIG. 3(f), the top of the tab moves back on the left side from underneath the lower edge 51 for release. When this pushing pressure is released after the above, the side wall, as deflected as shown in FIGS. (g) and (h), returns back in place while the tab 4 enters the slot 5. At this time, the lower face of the tab comes in contact with and is fixed on the upper face of the lower edge 51 of the slot.
Next, operations for opening the lid 2 are explained in the following. The side wall on the tab 4 side of the case body 1 is again pushed with a fingertip to the inside of the case body while deflecting this part and releasing the tab, the tab is disengaged from the slot 5, thus the lid becomes openable.
A lid-fixing structure, shown in FIGS. 3 (e).about.(h), is advantageous in that operations for fixing or releasing the lid can be performed at one touch. However, the structure still retains the following two defects.
The first defect is explained now. Operations for engaging with and releasing the slot 5 should be conducted smoothly without excess friction during deflection caused by pressing the wall on the tab side of case body 1. For this purpose, this slot must be drilled beforehand with a considerably large hole having sufficient width in relation to the size of the tab. In consequence, the size of the slot 5 becomes fairly large to allow insertion inside and outside of the case as shown in FIGS. 3 (g) and (h) with the lid closed. Therefore, the sealing of the case is greatly damaged.
Regarding the second defect, it is required to set the length of protrusion for the tab 4 to be rather long, namely the length of inserting the tab into the slot 5 in order to maintain firm fixation of the lid. As a result, it has been required to make the deflection quantity of the side wall rather large in the tab of the case body due to pressing for releasing or releasing operation of the lid.